Abstract : The report discusses investigations which have been carried out to study the process of strain ageing caused by oxygen in molybdenum by measuring the variation of the height of the internal friction peak with ageing time. It was found that the peak height decreases gradually and eventually disappears with ageing-time both in quench-aged and in strain-aged specimens. Systematic studies have been made on the kinetics of strain ageing and on the effect of deformation on the peak height. According to experimental results, it is believed that the decrease of the peak height is associated with the segregation of oxygen atoms to dislocations during ageing. On the basis of the assumption, the dislocation density and the atmosphere concentration were estimated. (Author)